To reduce engine emissions, it is desirable to rapidly heat catalytic converters in the engine exhaust. In other words, catalytic converters achieve higher emission reduction after they have reached a predetermined operating temperature. Thus, to lower vehicle emissions, various methods are available to raise catalyst temperature as fast as possible.
In one approach, during the warm up phase of the catalytic converter, the engine ignition timing is retarded, and the engine air-fuel combustion mixture is set lean of stoichiometry. In this way, the retarded ignition timing provides additional heat to the engine exhaust, while the lean air-fuel mixture produces minimal hydrocarbons. In this way, catalyst temperature is rapidly increased, while minimizing hydrocarbon emissions. Such a system is cited in U.S. Pat. No. 5,497,745.
The inventors of the present invention have recognized that a disadvantage with the above approach is that the amount of ignition timing retard is limited by combustion stability. This, in turn, limits the maximum amount of heat that can be added to the catalytic converter and thereby limits the ability to increase catalyst light-off times.
Another approach to providing heat in the exhaust is to operate the engine with a rich air-fuel mixture while providing additional air directly into the engine exhaust. Such a system is described in U.S. Pat. No. 5,136,842.
The inventors herein have also recognized a problem with the above approach. In particular, the inventors have discovered that increased exhaust emissions may result due to incomplete burning of the excess hydrocarbons and injected airflow in the exhaust system. In other words, if the exhaust temperature is too low, complete burning in the exhaust manifold upstream of the catalytic converter may not occur. Also, the inventors herein have recognized that the engine fuel economy suffers while operating the engine rich and providing the additional air directly into the engine exhaust. In other words, the excess fuel added to provide a rich air-fuel ratio (and which provides heat via an exothermic reaction in the engine exhaust) detracts from vehicle fuel economy.